Ab initio study of adsorption and diffusion of Ag atoms on a Si(001) surface

By employing ab initio total-energy calculations we have studied adsorption and diffusion of Ag atoms on a dimer-reconstructed Si(001) surface. For a single Ag adsorption, the twofold-coordinated cave site above the fourth Si layer atom was found to be the most stable, in agreement with previous works. Inspection of the electronic structures at the cave site revealed that the Ag-Si bonds originate from low-lying 4d electrons and are covalent. Our calculations also exhibit another stable adsorption at the pedestal site that is slightly higher by 0.03 eV in energy than the cave site. Further potential-energy-surface calculations showed that the surface diffusion of a single Ag adatom is unexpectedly highly isotropic and that the energy barrier is 0.5 eV. When more Ag adatoms are adsorbed on the Si surface, the Ag adatoms are expected to form dimers. Actually, we obtained an energy gain of 0.36 eV/dimer through the dimerization. The diffusion of an Ag dimer was also investigated. Surprisingly, we found a very rapid surface-dimer diffusion with an energy barrier of 0.48 eV slightly lower than that of the single Ag adatom. In contrast to the diffusion of the single Ag adatom, the dimer diffusion is anisotropic and preferably occurs along the valley between Si dimer rows by concerted flipflop processes.